In the industry, bulk solid powder products, comprising fine granules, are in big quantities packed up in sacks of 5 to 50 kilograms. Examples of products of that kind are cement, the so-called dry-mixes including cement or lime powder (e.g. dry-mortar, dry concrete-mix), limestone-powder, lime-hydrate powder, polymer powders (e.g. suspension poly-vinylchloride, S-PVC) etc. For the bagging of these products, easily mixing with air, typically paper sacks of porous walls, particularly valve bags, are used. It would be cheaper to use plastic packaging film for the automatic bagging of the fine powders, e.g. cement mixes. That can be done, for example, with the packaging machine called “Compacta for Cement”, made by Italian company “BL Bagline”, representing the state of the art. Still, the packaging of fine powders into plastic sacks has not become widespread. We have found that one, maybe the most important, reason thereof is that if fine granulated products are packed in traditional plastic film bags, then the stability of the stacks built from the bags is insufficient, the bags slip apart. That is, on the one hand, caused by the coefficient of friction of the smooth surface of the ordinary plastic film being generally lower than that of the usual paper grades, especially if the film is polluted with fine dust. On the other hand, the wall of the plastic film bag, unlike that of a paper bag, is not porous. Therefore a certain airing can at most be provided with perforating the surface of the film, during the filling-in of the powder-air mixture. A relatively fast airing can usually be achieved with vent hole perforations of at least about 3 to 4 mm's, but even that is less than sufficient. Because of the insufficient airing, the bags remain “inflated”, containing surplus air, even when they get into, and become a part of, a stack of bags. There are air cushions left in the inflated bags lying upon each other which, in the case of plastic films used so far for this purpose, makes the stack unacceptably unstable and the bags slip up on each other and fall from the pallet. All that prevents inexpensive plastic packaging films from being used with products of fine granules.
There has traditionally been an endeavour to decrease the slip of plastic bags by increasing the coefficient of friction of the film of the bags. Therefore, in accordance with the solution of the company Nordenia Kunststoffe, published in document DE 3437414A1, the wall of the plastic bag has been strongly embossed, from inside out, thus forming in the film hollow protrusions protruding to the outside. There has been an endeavour to form high and sharp protrusions by embossing the film which, however, weakens the film. According to our own measurements, such a strong embossing may decrease the breaking strength of the film by up to 14.5%, which increases the danger of the bursting of the inflated bag being in the stack. Therefore we deem embossing a disadvantageous solution in this field of packaging. At the same time, however, in a heavily dusty environment the beneficial effects of the embossing to the coefficient of friction of the film are negligible. The coefficient of friction of plastic films easy to emboss, rich in special low density polyethylenes, is usually low anyway. In addition, the hollow embossing, under the heavy load of the cement bags, will soon flatten out and get planar.
It is, on the other hand, known that U.S. Pat. No. 6,444,080, originating from the present applicants, is related to plastic films, of decreased slip, for making sacks between which the clinging is not primarily provided by the usual static coefficient of friction. In that solution at least one of the sliding film surfaces is rough, and its roughening protrusions interact with an engaging element of a loose, fibrous structure, practicably with an inexpensive nonwoven fabric, in a way by which a bond, of a strong shear strength, can be provided between the engaging element and the rough film or films. The essence therein is that the antislip protrusions are capable of penetrating between the free filaments, therefore the adhesion is not only based on the coefficient of friction of the materials but rather on a kind of a mechanical lock effect. According to the document (and to our own experience) the adhesion can be further increased if the antislip protrusions are undercut, i.e., their projection to the surface of the film is bigger than their intersection with the plane of the film.
This kind of film-roughening has, over other kinds of film roughening methods, the distinguishing feature that the roughening protrusions are formed with fixing powder granules or other, essentially point-like, particles to the film and preferably have a typical undercut. Thus these protrusions are not mere embossed protrusions but they add extra material to the film therefore they do not essentially weaken the film. These protrusions are thus of essentially point-like topology in comparison with the protrusions of such other kinds of roughening in which the protrusions are long, straight lines or ridges and valleys of linear topology, winding in a random manner.
In our experience the aforementioned antislip solution, comprising rough film and engaging element, has worked very well and reliably in practice with skidproof packaging of pellets, and it can be used in several ways. In one possible solution at least the upper surfaces of the sacks, laid upon each other, are rough and at least their lower surfaces are provided with an engaging element, for example a nonwoven fabric, fixed thereto (the roles of the upper and lower surfaces can be inverted). The engaging element can be fixed to the sacks before, during or after the filling. Here the sacks must be stacked in a brick-like bond. In another solution both the lower and upper surfaces of the bags are rough, and the engaging element, for example nonwoven fabric, is present in the form of a stick-sheet laid between the layers of bags. The latter arrangement provides the advantage that the bags do not have to be stacked in a brick-bond pattern but they can also be arranged in columns and that in many cases it is not necessary to apply an engaging sheet to every layer (source reduction). It is a further advantage of the latter arrangement that the different polymer materials of the system (nonwoven fabric and bags) can easily be separated with a view to a recycling.
The requirement of a good printability of the rough sack surface prompts the skilled person to use antislip protrusions as small and as low as possible. The same urge originates from a further advantage of the lower protrusions, that the surface of such bags is more comfortable to touch, is not so rough, which is very important for many smaller industrial users (for those whose workers move the bags by hand and a too abrasive rough film surface might hurt their skin).
That method or such film bags are not known, from the prior art, to be used in the field of packaging fine powder products.